1. The Theory of Flight Measuring Drag
BTEC National Programmes in Aerospace Engineering
The Theory of Flight
Measuring Drag

2. Warm up quiz Which force opposes weight? Which force opposes thrust?
What is the formula for calculating lift?
What is the formula for calculating drag?
Name the two main classes of drag?
For an aircraft we can use L/D as an important ratio. For an airliner do you think we should have a low or high figure here? Why?

3. BTEC National Programmes in Aerospace Engineering
The Wright brothers, who pioneered powered flight, also engineered a device to measure the drag produced by an aerofoil.
This device was first called a ‘drift balance’ but is now referred to as a Drag Balance.
The Wright brothers used this device to find which aerofoil sections produced the most lift and least drag or, in other words, which designs were most efficient.

4. BTEC National Programmes in Aerospace Engineering
Wind Tunnels
Like the lift balance, the device that the Wrights made works in a wind tunnel.
The mechanism measures the drag force produced by the aerofoil.

5. The amount of movement (i.e. drag) is shown by the moving pointer.
BTEC National Programmes in Aerospace Engineering
The Drag Balance
Airflow
Airflow passing over the aerofoil creates a drag force that makes the aerofoil move backwards.
The amount of movement (i.e. drag) is shown by the moving pointer.

6. The Drag Balance Mechanism (top view)
BTEC National Programmes in Aerospace Engineering
The Drag Balance Mechanism (top view)
The parallelogram of links A,B,C,D pivots about fixed points P and Q.
The airflow over the aerofoil creates a total reaction force (TOTAL FORCE) due to the lift and drag produced by the wing.
The drag force cause the parallelogram to move left – link D rotates around pivot Q and the pointer that is fixed to D rotates with link D.
The angle moved through by the pointer () depends on the amount of force (and therefore drag) produced by the aerofoil.

7. BTEC National Programmes in Aerospace Engineering
The Open University has also produced a ‘virtual drag balance’ based on the Wright drift balance design.
It can also be found at:

8. Click on ‘simulations’
BTEC National Programmes in Aerospace Engineering
Click on
‘simulations’

9. Click on ‘drift balance’
BTEC National Programmes in Aerospace Engineering
Click on
‘drift balance’

10. simulation’ to use the balance.
BTEC National Programmes in Aerospace Engineering
Play the movie
then click on
‘shockwave
simulation’ to use the balance.
Refer to the work sheet ‘Measuring Drag’

11. Results Angle of Deflection (Aerofoil 35) 2.5 5 10 15 25 45 20.5 8 3
2.5 5 10 15 25 45
20.5 8 3 -2
0.5 1
-0.25

12. From the data we can calculate:
CD – coefficient of drag – this is a multiplication factor to establish overall drag
Using the spreadsheet
Your Name:
Aerofoil No:
Angle of Attack
Deflection Angle
Glide Angle
Cd/Cl
Cl/Cd
Cl (from lift bal) Cd
#DIV/0!
2.5 5 10 15 25 45 1
Enter data for deflection angle and Cl (from Lift Balance experiment) for each angle of attack
You should plot for one aerofoil:
angle of attack v's Cd
lift v's Cd/CL
From previous lift exercise
From Drift Balance Data
This can be found on blackboard

13. Exercises
b. Carry out wind tunnel experiments as detailed in the attached worksheet ‘Measuring Drag’ and produce drag plots and L/D ratio plots for a selected aerofoil section.
You should create plots of:
Angle of attack v’s CD
Lift v’s CD/CL (Criteria P5)
3. Research and draw a drag/airspeed graph and use it to explain how zero lift (or profile drag) and induced drag vary with airspeed. By including total drag, show when minimum drag is achieved and explain the significance of this value.
(Criteria M1)